The present invention relates to actuation circuits for high-current switching devices such as relays or contactors, that is, devices in which the appearance of a pilot current or voltage causes the opening or closing of a controlled switching device. This invention is more particularly concerned with a simple and inexpensive actuation circuit which ensures smooth operation of a relay actuator coil over a wide voltage range without chatter and without overheating.
Electromechanical relays are electromagnetic devices in which current flowing through a coil actuates (i.e., closes or opens) a pair of electrical contacts. This can occur in a number of well known ways, but usually an iron armature is magnetically deflected towards a soft iron core of the coil to make (or break) the controlled circuit.
Each relay is designed for a fairly specific range of operating voltages. For example, a relay may be designed to operate at a nominal 15 volts, in which case it may have a design operating range of about 12 to 18 volts. At voltages much below twelve volts, the current through the actuator coil may be too small to close the relay contacts, or may cause the contacts to close intermittently and "chatter." On the other hand, at voltages much above eighteen volts, the increased current through the coil can cause overheating or coil burn-out, and also represents wasted energy.
In some designs with DC relays, a wide operating voltage is required, but a specific relay must be specified for the circuit design. If the relay selected is of a nominal voltage, it may chatter or buzz on pick up if the voltage is too low. This can cause excessive arcing of the relay contacts, and could potentially weld or damage the contacts. On the other hand, if the relay is selected of lower than nominal voltage, it may be overstressed when the voltage is high but is still in the expected range.
One possible solution is to employ a voltage regulator circuit to keep the relay voltage constant. However, the voltage regulator itself consumes energy, and also requires heat sinking for power dissipation.
Furthermore, in many environments the voltages applied to these relays have varied far more than was originally expected. For example, in a recreational motor home environment, because of unusual wiring, load, and power production conditions, a nominal 117 volts (RMS) has appeared as low as 63 volts and over 160 volts (RMS). When converted in a nominal 24 volt (5:1) thermostat transformer, the corresponding voltage range is from only 13 volts to above 33 volts.